This invention relates to the field of high voltage electrical distribution equipment, more particularly to the control and transfer of high voltages to ultimate loads through electrical transformers, and more particularly to mechanisms used to control transformer secondary circuit voltage under varying electrical loads and to prevent arcing during the selection of winding taps.
The typical electric distribution system includes a power source, such as a hydroelectric dam or a coal or nuclear fired generating station, a high voltage three-phase distribution system, and line transformers to step down the distribution line voltage to a value acceptable to the end user.
To reduce power loss caused by the resistance of the distribution power lines, the main power transmission lines emanating from the power source to the local power substation typically carry voltage potentials in excess of one hundred thousand volts. However, electricity at such high potential is unsuitable for almost all industrial and residential use. Therefore, the voltage is stepped down at a substation adjacent the user of the electricity by a power transformer. The output of the power transformer will typically be on the order of 1300 volts. Electricity of this voltage is then supplied on power lines to industrial and residential areas, where further transformers may be used to lower the voltage to 440 volts, 120 volts, etc.
The power transformer is constructed having a high voltage winding, a secondary winding and a magnetic core. The high voltage winding consists of a wire wound in a series of wire loops around the core, the ends of which are connected to the high voltage distribution system. The secondary winding is likewise comprised of a series of wire loops wrapped around the metal core. The secondary winding has a far fewer number of windings than the high voltage winding. Thus, the voltage induced on the secondary winding is far lower than that on the high voltage winding. The secondary winding is connected to the ultimate local load distribution system.
Although, because of the effect of line losses and other non linear effects, the ratio of primary to secondary coil windings does not exactly match the ratio of input or primary voltage to output or secondary voltage, the correspondence is close enough to permit fine voltage regulation on the secondary voltage side of the transformer by making slight modifications in the number of secondary windings which are in conductive engagement with the load. This is accomplished by placing a series of leads, or taps, in conductive engagement with the secondary coil at an evenly spaced number of windings apart. For example, if a ten percent variation were required, a tap would be placed on the transformer secondary coil at approximately ten percent of the windings from the end of the secondary coil. Further refinement in that ten percent variation may be accomplished by further subdividing the final ten percent of the windings with additional taps.
If the load on the secondary circuit varies, it can cause the voltage in the secondary circuit to likewise vary. For example, if the load increases, the voltage in the secondary circuit will decrease. Likewise, load decreases in the secondary circuit will increase the voltage in the secondary circuit. Such variations in line voltage can be detrimental to the performance and life of industrial equipment, and annoying to residential electricity users.
To address the load voltage variation, a load tap selector is used. A load tap selector is a device which employs a secondary circuit voltage detector which actuates a mechanical linkage to selectively engage the winding taps with the secondary circuit in response to load variation induced secondary circuit voltage variation. The load tap selector typically contains a triplicate set of parts to induce tap changes on all three phases of the three-phase circuit.
One common load tap selector is a rotary load tap selector. This is a mechanical device which selectively engages the winding taps by actuating rotary tap selector arms which conductively and mechanically engage metal clips which are in turn wired to the winding taps. One part of the rotary selector arm engages the metal clip, while another part maintains engagement with a slip ring which is wired to the load circuit. The selector includes three pairs of coaxially disposed rotary selector arms, each of which is engaged with one of a pair of Geneva gears. The engagement of a selector arm to a specific tap winding clip completes an electric circuit from the tap winding through slip rings to the load circuit on a phase. The tap winding clips are equally arcuately disposed in a circle about a slip ring so that rotation of the selector wheel in specific arcuate steps creates an electrical path through the specific tap winding to the secondary circuit through the slip rings.
To prevent excessive arcing when the tap winding clips are engaged and disengaged, the tap winding selector includes a pair of split switching reactors, a vacuum interrupter disposed between the reactors, and a pair of bypass switches disposed between the reactors and the loads. The bypass switches and interrupter open the circuit between a tap and the load, which prevents arcing, as the tap selector arm disengages a tap and engages an adjacent tap.
Typically, the tap selector includes two rotary selector wheels engaging tap clips electrically connected to the tap windings to select one of a range of voltages of plus or minus ten percent of rated secondary circuit voltage in 5/8 percent voltage steps. Therefore, for a plus or minus ten percent voltage variation, nine tap clips are disposed in a circular pattern around the slip ring for engagement by contacts connected to the selector wheels. One of the tap clips is a neutral tap clip. A reversing switch is employed to permit each tap clip to be selectively engaged with one of two sets of taps equally disposed from the neutral tap clip. The neutral tap is located at the rated voltage winding on the secondary circuit. The switch connects the tap clips to the high voltage side or the low voltage side of the neutral tap. Where the neutral tap is located at ten percent of the windings from the end of the transformer, this configuration permits sixteen voltage changes per wheel for a total of thirty-two stepped voltage changes and a total percentage variation of twenty percent.
In the neutral, or rated output winding position, both rotary selector wheels engage the neutral tap at the neutral position and also engage the two slip rings. To increase the number of effective windings on the high voltage side of neutral, the first selector arm is moved clockwise to the first tap selector clip, adding energized windings to the secondary coil to increase the secondary voltage 5/8 percent. To further increase the effective number of windings, the second selector arm is moved counter-clockwise to the first selector clip further adding energized windings to the secondary side of the transformer, and at the same time flipping the reversing switch. Successive increases in voltage are effected by further movement of the selector wheels until both wheels engage the eighth or last tap selector clip. To decrease the voltage on the high voltage side, the arcuate movement of the wheels is reversed (clockwise), until the original neutral position is regained. In moving from the first position back to neutral, the reversing switch is again flipped. Each reverse step, while the reversing switch is located to link the taps on the high side of the neutral tap, results in an output voltage reduction.
To decrease the voltage on the low voltage side of the neutral position, the reversing switch must be changed to the low side. As referenced above, this is accomplished as the tap clips regain the neutral position. When the reversing switch is flipped, the tap clips are connected to a second set of winding taps disposed in an equal and opposite direction from the neutral position. Then subsequent counter-clockwise motion of the wheels to arcuately actuate the tap clips reduces the effective output voltage. Selection of the appropriate voltage is effected by arcuately rotating the selector wheels to the proper tap winding clip.
The structure of the rotary tap selector is such that maintenance to assure gear synchronization is absolutely essential. If one of the selector arms engages the neutral tap winding while the remaining selector arm is engaged on the eighth tap, the transformer will short circuit across the secondary winding resulting in complete transformer failure. This will occur if both arms are engaged on the eighth tap, and selector arm progress one step to the neutral tap. In the past, the only means of preventing this condition was vigilant maintenance to assure that all parts were synchronized to maintain proper alignment. However, improper maintenance, as well as long term wear of the load tap changer components, can result in misalignment and transformer failure. The present invention overcomes the deficiencies of the prior art.